This invention relates to a process and apparatus for regenerating fluidized cracking catalyst.
The field of catalytic cracking, and particularly fluid catalyst operations, has undergone significant development and improvements due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalysts, and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered, requiring even further refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in a fluid catalytic cracking (FCC) installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition generally upwardly through one or more riser conversion zones (fluid catalytic cracking zones), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature risers and 0.5 to 4 seconds hydrocarbon residence time in the riser may be used to make cracked products.
During cracking carbonaceous deposits accumulate on the catalyst. Entrained hydrocarbons are removed from the catalyst in a stripping zone. Cracked products and stripped materials are combined and typically passed to a product fractionation step. Stripped catalyst (spent catalyst) containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then regenerated.
In catalyst regeneration, oxygen burns off coke. The hydrogen-containing components in coke form water which causes hydrothermal degradation.
U.S. Pat. No. 4,336,160 to Dean et al attempts to reduce hydrothermal degradation by staged regeneration. However, the first stage of the regeneration process of Dean et al employs a dense bed which provides an opportunity for hydrothermal deactivation.
A major trend in fluid catalytic cracking processing has been modifications to the process to permit it to accommodate a wider variety of feedstocks, in particular, stocks that contain more nitrogen than had previously been permitted in a feed to a fluid catalytic cracking unit.
Many FCC feeds contain a lot of nitrogen. There is a trend to heavier, dirtier feeds. There is also a growing concern about the amount of NO.sub.x in the regenerator flue gas. Some attempts have been made to minimize the amount of NO.sub.x discharged to the atmosphere through the flue gas by employing multiple beds in a fluid catalytic cracking regenerator.
U.S. Pat. No. 4,325,833 to Scott discloses a three-stage regenerator directed to NO.sub.x removal. Scott discloses that his middle stage contains a substantially oxygen-free atmosphere to convert NO.sub.x to N.sub.2. However, flue gas from lower beds contact with catalyst from upper beds. This is detrimental because the flue gas contains water which can deactivate the catalyst by hydrothermal degradation.
It would be advantageous to provide a process which both minimizes NO.sub.x and hydrothermal degradation.
Accordingly, the present invention provides a process for fluidized bed regeneration of coke contaminated catalyst by combining a stream of coked catalyst with a stream of hot regenerated catalyst and a first oxygen-containing gas stream to form a first mixture of catalyst and gas and regenerating the catalyst by burning the coke characterized by passing the mixture through a first stage regenerator comprising a first regenerator riser having an upper end and a lower end and maintaining a low oxygen concentration in the upper end of the riser; discharging the first mixture from the riser to form a first catalyst bed located in a lower portion of a second stage regenerator; adding a second oxygen-containing stream to the first catalyst bed to form a second mixture of catalyst and gas which passes to an upper portion of the second stage regenerator and maintaining a low oxygen concentration in the upper portion of the second stage regenerator; discharging from the upper portion of the second stage regenerator catalyst with reduced coke content and flue gas; and recycling to the first stage regenerator a hot regenerated catalyst stream obtained downstream of the second stage regenerator.
FIG. 1 is a partial cross-sectional view of a regenerator of the present invention;
FIG. 2 is a partial cross-sectional view of a second embodiment of a regenerator of the present invention;
FIG. 3 is a partial cross-sectional view of a third embodiment of a regenerator the present invention; and
FIG. 4 is a schematic of a fourth embodiment of the invention.